[71.20] ``Coherent Cores" as the Sites of Turbulent Stream Collisions

It has been found recently that small (\lesssim 0.1 pc)
dense cores in molecular clouds show a non-thermal velocity
dispersion which is independent of size (Goodman et al
1998). We suggest that, rather than being an indication of
velocity coherence, such a flat scaling may be interpreted
as the signature of the collision between larger-scal gas
streams. This is verified in MHD numerical simulations of
the ISM. If the relative velocity difference between the
streams is not very large, then the mass-weighted velocity
histograms, which are the numerical equivalent of optically
thin spectra, show a single spectral component of nearly
constant width over a range of scales within the core, as
long as the shock site remains unresolved.

In this escenario, a core with a size-independent velocity
dispersion is not decoupled from its environment, as has
been suggested previously. Instead, it is causally and
dynamically connected with the external flow.